Highly Pure Stainless Steel Process

ABSTRACT

This invention discloses a proper process control to produce a highly pure stainless steel cable products, including electro skim to improve smoothness of the rough surface of said product, electrolyte bath, electropolishing for leveling of roughness on the surface of said product and for increased resistance to corrosion, activation with weak acid, bath cleaning, and drying.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electrochemical polishing process of stainless steel; and more particularly, it relates to a proper process control to produce highly pure stainless steel cable products with a brilliant appearance, an improved resistance to corrosion, and an enhanced life cycle.

2. Description of the Prior Art

Most stainless steel cables in traditional metal finishing industry are manufactured for a pre-determined degrees of strength by going through the skimming process, which often leaves mechanical deformation on surface layers of stainless steel produces, such as a rough surface, which in turn shortens life cycle of cable products in long-term exposure to the outside world. Furthermore, traditional skim processes are unable to completely remove residuals, making surface of the stainless steel cable products yellowish and less shining.

In general, a film of chromium oxide is often added to the surface of a film of chromium most stainless steel cables to protect them from corrosion quickly. As a result, thickness of the film of chromium (added to the surface of the products) is directly related to the stainless steel product's resistance to corrosion. Result of AES (Auger Electron Spectroscopy) analysis shows that most stainless steel cables are covered with only a thin layer of chromium oxide film, as reference 1 indicates; Still, addition of such a thin layer nevertheless adds manufacturing cost, as reference 2 shows. A third disadvantage of adding a thin layer of chromium oxide film on the surface of the products is that scales may be produced as a result of compression with eye layer in high temperature during the threading process.

SUMMARY OF THE INVENTION

The main object of the invention is to improve the above-mentioned defects produced during the skim process of stainless steel threads or cables in traditional industry. Smoothing-enhanced surface process is added in this invention to stainless steel products to increase products' resistance to erosion and hence their life cycle.

The first object of the invention is to apply a series of processes to products of stainless steel threads or cables, including electrolysis skim, bath cleansing, electropolishing, acid activation, bath cleansing and drying, aiming to improve the surface smoothness and polishing, and hence their resistance to buildup, and contamination as well as corrosion resistance.

The second object of the invention is to apply a major ingredient, 30 g/l to 60 g/l of sodium hydroxide during electro skim process of stainless steel threads or cables.

The third object of the invention is to apply a major ingredient, 40% to 60% of phosphoric acid and 20% to 40% of sulfuric acid, during electropolishing process of stainless steel threads or cables.

The fourth object of the invention is to apply a major ingreident, 5% to 10% of nitric acid during weak acid activation process of stainless steel threads or cables.

Advantages of the current invention are summarized below:

[1] Stress Free Polishing (SFP): Electropolishing is accomplished by connecting the metal part to be processed to the positive terminal (the anode) of a DC power supply. The object to be electro-polished is immersed in an electrolyte and subjected to a direct electrical current. Metal ions dissolved from the annode react with phosphoric acid in the polishing solution to form a film at the surface, which has the property of low dissolvibility and high viscosity and accumulates around the annode; the film is thinner over the micro-projections and thicker at the micro-depressions, resulting in a more rapid dissolution of the micro-projections causing micro-leveling at the surface.

[2] Improved physical appearance: excellent light reflection and depth clarity, bright, smooth polish; uniform luster of shaped parts.

[3] Enhanced mechanic properties: less surface drag and friction, increased duty cycle, and decreased scaling and product build-up.

[4] Increased resistance to corrosion: increased pitting potential and resistance to corrosion due to the surface oxide layer, which is rich in oxygen and chromium and poor in iron.

BRIEF DESCRIPTION OF THE DRAWINGS AND REFERENCES

FIG. 1 is a flow chart illustrating the cleansing and polishing process in manufacturing stainless steel cables disclosed in this invention;

FIG. 2 is an illustration of the electropolishing process disclosed in this invention to smooth out surface of stainless steel cables disclosed in this invention;

Reference 1 is a chart showing ingredients amounts contained on AES surface of most stainless steel cables in traditional metal finishing prior arts;

Reference 2 is an SEM figure showing scratches during threading process of most stainless steel cables in traditional metal finishing prior arts;

Reference 3 is an SEM figure showing oxidized scales generated during threading process of most stainless steel cables in traditional metal finishing prior arts; and

Reference 4 shows ingredients amounts contained on AES surface of stainless steel cables after electropolishing process disclosed in this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 and 2, this invention discloses the structure of a highly pure stainless steel cables and its manufacturing process. To effectively execute the skimming and electropolishing process for brightening appearance and improved property against corrosion, step-by-step procedures are listed in order below:

-   -   (1) Products made of stainless steel threads or cables are first         electro-skimmed to achieve a complete skimming result;     -   (2) Products made of stainless steel threads or cables are then         immersed in a electrolytic bath;     -   (3) Products made of stainless steel threads or cables are then         electropolished to smooth out the rough surface for improved         resistance to corrosion;     -   (4) Products made of stainless steel threads or cables are then         activated by weak acid;     -   (5) Products made of stainless steel threads or cables are then         bathed; and     -   (6) Products made of stainless steel threads or cables are         dried.         -   The major ingredient used for step (1) process of             electro-skimming is 30 g/l to 60 g/l sodium hydroxide.

During step (3) process of electropolishing of stainless steel cables made of stainless steel threads with twisting process, stainless steel cables are immersed in an electrolyte composed mainly of phosphate (to be more specific, the electrolyte consists of 40% to 60% of phosphoric acid and 20% to 40% of sulfuric acid), and subjected to a direct electrical current. With the movement of both positive and negative ions caused by the electrical reaction, particles of metal are removed.

After electropolishing, the laboratory corrosion test of Anodic Polarization at 3.5% wtNacl shows that the potential against surface corrosion increases significantly, indicating that the oxidized layer on the surface of the stainless steel cables is more resistant to corrosion. Furthermore, the annual corrosion rate (mm/year) also drops from 0.07 (mm/year) prior to polishing to 0.03 (mm/year) after our step-by-step procedures of polishing disclosed in this invention.

In the meantime, the surface smoothness due to a complete skimming brings a brightening appearance to the stainless steel cables, too.

The outer layer of the stainless steel cables after our polishing process contains more in chromium and oxygen. The oxygen amount contained in the most outer layer is 40%, which decreases from outer to inner layer, and is maintained at 10% at the inner layer, indicating more chromium and oxygen are contained in the stainless steel cables after our polishing process (please also refer to Reference 4, which shows ingredients amounts contained on AES surface of stainless steel cables applied with our electropolishing). The thickness of the oxidized layer of with the stainless steel cables with our electropolishing is estimated to be at least 5 times that of the oxidized layer without our electropolishing, based on the amount of oxygen contained in the layer.

No mechanical deformation (i.e. rough surface) is observed on the surface of the stainless steel cables under the SEM microscopic view at 1000× power, which greatly reduces the cable's corrosion.

The main ingredient used in step (4) activation process with weak acid is 5% to 10% of Nitric acid.

Although the preferred embodiment in accordance with the present invention has been provided in this application, it is to be understood that many other possible modification and variations can be made without departing from the scope of the present invention hereafter claimed. 

1. A process for a product of said structure of highly pure stainless steel cable, including electro skim to improve smoothness of the rough surface of said product, electrolyte bath, electropolishing for leveling of roughness on the surface of said product and for increased resistance to corrosion, activation with weak acid, bath cleaning, and drying.
 2. The process as claimed in claim 1, wherein the major ingredient used in the process of said electro skim includes 30 g/l to 60 g/l sodium hydroxide
 3. The process as claimed in claim 1, wherein the major ingredients used in the process of said electropolishing include 40% to 60% of phosphoric acid and 20% to 40% of sulfuric acid.
 4. The process as claimed in claim 1, wherein the major ingredient used in the process of said activation with weak acid includes 5% to 10% of Nitric acid. 